Page:The American Cyclopædia (1879) Volume XI.djvu/532

 MICROSCOPE carefully used, surpasses all but the most im- proved forms of the compound instrument. Doublets of gems are far superior to those of glass. Triplets are superior to the doublets ; and for a simple microscope the achromatic triplets now furnished by the French and Ger- man opticians, as objectives for the compound microscope, will be found very effective. Es- sentially, the English and American achromatic objectives are triplets, but the peculiar mount- ing of these instruments prevents their use as simple microscopes. So great has been the improvement, that the best modern objectives will transmit angular pencils of 170 to 178. We may notice here the so-called " Codding- ton lens," or grooved sphere. It is an inven- tion of Sir David Brewster, and when proper* ly made is almost free from spherical aberra- tion, and the chromatic aberration is almost insensible. "It consists of a spherical lens, or sphere with a deep concave groove cut round it, so as to cut off the marginal pencils, and thus give a wider field and more perfect image." The lenses usually sold under this name are simply cylinders of glass having spherical ends, and of course have none of the advantages of large field and freedom from spherical aberration proposed by Dr. Brewster. When the curvatures of this cylindrical lens are unequal, and such that, the most convex being turned toward the eye, an object placed on the other convex surface is in the proper focus of the lens, it is called a "Stanhope lens ;" its use is limited to such objects as can be directly applied to the surface. When of considerable power it may be advantageously employed in searching for diatomacece; the drop of water supposed to contain them may be examined by applying it to the less convex surface. All the simple microscopes, and es- pecially the higher powers, require some kind of a stand or carrier. The lower powers and single lenses are usually attached to the end of a jointed rod, which can be moved up and down a stem inserted into a solid base. The most convenient mounting for an inch or half- inch lens, for preliminary examinations or bo- tanical dissections, is that of Messrs. Powell and Lealand of London, and is employed as the mounting of the small condenser for their com- pound instrument. The movements are com- plete, and one can place the lens, whatever may be the position of the object, in such a relation to it as will insure the best view. For the higher powers, Jto T V of an inch, a steady well made stand will be required, and some means of adjusting the focus delicately, either by rackwork or screw. Various forms have been devised ; perhaps, upon the whole, that known as the " Raspail " is most simple, and at the same time of great excellence. It con- sists of a brass pillar, up and down which a large circular stage is moved by rackwork ; a large mirror, one side plane, the other concave, swings freely below, and serves to direct the light upon the object ; at the top of the brass pillar is placed the lens holder, movable for- ward by means of a screw, and laterally by swinging round a pin inserted in the top of a pillar ; into the opening of the stage is fitted a glass plate, or it may be made to hold dissect- ing troughs with glass bottoms. It is often convenient, or absolutely necessary, for the examination and dissection of opaque objects, to have the lens inserted in a eilver cup or Lieberkuhn, which, receiving the light from the mirror below, reflects it back, condensed, upon the object. These Lieberkuhns are usu- ally made of silver. The very simple micro- scope employed by Ellis in his researches on coralline, in which all the adjustments were effected by sliding by the hand, was fitted with these silver cups. Although the Lieberkuhn is very commonly applied to the low power achromatic objectives, it is now seldom to be obtained with any form of simple instrument, unless by special order ; it will be found of the greatest service in minute dissection. In using lenses of moderate focus, three fourths to one eighth inch, the most extended distinct field is obtained when the convex side is presented to the object ; but the sharpest vision of a mi- nute point or small object, in the centre of the field, is when the flat side is presented to the object. In estimating the magnifying power of single lenses, an arbitrary standard of the nearest distance at which the healthy unassist- ed eye can view distinctly minute objects is assumed; this distance has been placed at from 5 to 10 in. The latter is the standard adopt- ed by most opticians and authors ; Sir David Brewster alone adopts 5 in. The magnifying power is obtained by dividing 10 in. by the solar focal length of the lens, and is usually expressed lineally, or as " so many diameters." Thus, when the magnifying power is stated to be 40, it is meant that the diameter is in- creased 40 times, but of course the area would be increased 1,600 times. The following ta- ble exhibits the linear and superficial magni- fying power, adopting the standard of 10 in. : Focal lengths in inches. power. Superficial magnifying power. 2 '5 26 1# 6-6 48-5 1 10 100 % 18-8 176-8 x 20 400 X 40 1,600 X 80 6,400 1 ,o 100 10,000 Vao 200 40,000 As it is difficult to measure exactly the solar focal length of small lenses, a sufficient approxi- mation may be had by the method proposed by Mr. Ross, which answers admirably for doub- lets and triplets. It consists in "viewing the image of some distant object formed by the lens in question, through another lens of one inch solar focal length, keeping both eyes open, and comparing the image presented through the two lenses with that of the naked eye.